Method To Achieve A Stiffened Curved Metallic Structure And Structure Obtained Accordingly

ABSTRACT

The invention relates to a process for making a reinforced curved metallic structure, made from a heavy-gauge or plate-gauge sheet of an aluminium alloy containing scandium. 
     The process consists in combining forming ( 200 ) and machining ( 300 ) steps, the forming step ( 200 ) giving the structure curvature(s) and the machining step ( 300 ) producing a network of ribs for reinforcing the structure. A finishing step ( 400 ) can complete the preceding steps. 
     Such a process makes it possible to obtain a monobloc metallic structure with good mechanical properties simply and quickly. Application to the manufacture of a reinforced curved metallic structure of an aircraft subassembly.

The invention relates to a process for making a reinforced or stiffened curved metallic structure, and also to the structure made by performing this process. The structure is said to be stiffened or reinforced in that it has ribs (also known as “stiffeners”) for reinforcing against deformation of the structure.

In general in the rest of the description, the term “thin sheets” means sheets less than 12.7 mm thick, sheets between 12.7 and 40 mm thick are referred to as “heavy-gauge” and sheets greater than 40 mm thick are referred to as “plate-gauge”.

Conventionally, this type of structure is made by rolling, dissolving at high temperature followed by hardening, drawing and maturing or tempering of elemental thin sheets formed from an aluminium alloy. Such a range of treatments allows the sheets to have radii of curvature suited to the conformation of the target structure. The sheets and stiffeners/reinforcers are then assembled by riveting to form the final structure. Thus, for example, for landing gear cases, the sheets are assembled as stiffened subassemblies riveted together. By mechanical, chemical or electrochemical machining of the thin sheets, material is removed from the surface so as to allow the structure to have the desired surface state. Reinforcers are also added by riveting to consolidate the sheet as a whole.

The dimensions of the target structures thus makes it necessary to assemble elemental thin sheets and to reinforce these sheets.

The current solutions have many drawbacks: a large number of manufacturing cycles of long duration relating to the assembly, riveting and reinforcer attachment operations, and generating high manufacturing costs. Furthermore, this assembly approach is detrimental to the freedom of design and adaptation of the structures.

Moreover, the approach consisting in making structures from heavy-gauge sheets made of the alloys usually used based on Al—Zn (AA7XXX) or Al—Cu (AA2XXX) leads to the production of pieces that have reduced mechanical properties following hot forming. The reason for this is that high temperatures may destroy the microstructure of the sheet, which is reflected by limited mechanical characteristics, as regards the tensile strength, the elasticity limit, the fatigue strength or the resistance to external pressures. On the other hand, during cold forming of these pieces, large internal stresses are created and are not or are poorly resorbed.

The invention is directed towards making integral metallic structures with sufficient mechanical properties so as to withstand external impacts and pressure.

To do this, the invention proposes to make structures of this type from a heavy-gauge or plate-gauge sheet of an aluminium alloy containing scandium. It has been notably found that a heavy-gauge or plate-gauge sheet formed from such an alloy can undergo shaping with high radii of curvature while at the same time maintaining satisfactory mechanical characteristics, by preserving the microstructures and while avoiding the generation of large residual stresses.

More specifically, one subject of the present invention is a process for making a stiffened metallic structure with a uniaxial or biaxial curvature in monobloc form referred to as an integral structure, which consists in machining a heavy-gauge or plate-gauge sheet of an aluminium alloy material containing scandium, for example by rolling or any other process for forming this material into the said sheet, and then in combining steps of forming and machining of the said sheet, the forming step giving the curvature(s) and the machining step producing a network of reinforcing ribs.

Under these conditions, it is no longer necessary to assemble several elemental sheets, since the monobloc form constitutes the integral structure.

According to particular embodiments:

-   the machining step is performed after the forming step; -   the machining step precedes the forming step; -   the machining step is 3D and 5-axis machining; -   the forming is hot forming; -   the hot forming is shaping with a punching and stamping tool heated     to the same temperature as the sheet, this tool possibly being     hollow and/or made of a ceramic material so as to minimize its     thermal conductivity and to avoid heat losses; -   the thermal contact between the die and the sheet to be formed is     minimized by dimpling on the edge of the die; -   the thermal exchange may also be minimized by preheating the mould     and/or punch; -   the hot forming is performed by pressing the punching tools onto the     die at a temperature of between 325 and 350° C. for a variable time     that may be, for example, up to 24 hours, the deformation then     resulting from creep; -   the hot forming is completed by retraction of the stamping tools     followed by slow cooling of the form obtained in the open air or in     a suitable chamber, so as to promote relaxation of the internal     stresses and to minimize their residual presence; -   the aluminium alloy is an Al—Mg—Sc (aluminium, magnesium, scandium)     alloy with proportions of magnesium and scandium, respectively, of     less than 8% and 0.5; -   the process may be completed by a step of finishing by     rectification.

The invention also relates to a reinforced curved metallic structure produced via the above process, especially for making an aircraft subassembly, in particular a landing gear case, an airtight underside (at the front end), an aircraft cockpit and an aircraft emergency exit.

Other data and advantages of the present invention will emerge on reading the description that follows, with reference to the attached figures, which show, respectively:

FIG. 1, a diagram summarizing the main steps of two embodiments of the process according to the invention;

FIG. 2, a half-view in schematic cross section of the implementation of an example of hot forming of a plate-gauge sheet according to the invention for an airtight aircraft underside;

FIG. 3, a view in schematic cross section of the implementation of an example for the 3D-5-axis machining of a plate-gauge sheet according to the invention for a landing gear case; and

FIGS. 4 a to 4 d, views in perspective of examples of metallic structures curved and reinforced according to the invention by implementing the process.

With reference to FIG. 1, the first step 100 of the process according to the invention consists in rolling a primary form made of heavy-gauge sheet (greater than 12.7 mm thick), or even plate-gauge sheet (for example between 40 and 250 mm), and formed from an alloy based on Al—Mg—Sc comprising, in the example, 0.025% of scandium. The proportions of magnesium and of scandium remain, respectively, less than 8% and 0.5%, preferably, respectively, between 2% and 7% and between 0.01% and 0.45%, and even more preferentially within the ranges 3-6% and 0.015-0.4%. Such an alloy is described in patent EP 1 682 688.

In the preferred embodiment illustrated in FIG. 1, the first preparation step consists in hot-forming (step 200) a primary form made of Al—Mg—Sc alloy thus forming the skeleton of the structure to be made.

The scheme in cross section in FIG. 2 more particularly shows an example of implementation of hot-forming of a plate-gauge sheet according to the invention to make a landing gear case. A 70-mm-thick rolled sheet 2 is placed between the male punch and the die 6. The tool is made of ceramic to minimize its thermal conductivity and thus to reduce the energy losses. The tools are preheated to the forming temperature, i.e. to about 330-340° C. The die 6 has on its edges 6 b a dimple 6 c which limits the thermal contact between the sheet and the die. Alternatively, it is possible to insert an insulating mattress.

The forming then consists in moving the punch 4 against the die 6 of complementary shape with appropriate force. The punch then exerts a pressure on the sheet 2. Under the combined effect of this pressure and of the temperature, the sheet 2 becomes deformed in a convex configuration and marries the shape of the die 6 on its outer face 22, and the complementary shape of the punch 4 on its inner face 24, the outer and inner faces being substantially parallel.

The sheet 2 is then rapidly removed from the forming tools. The opposite free edges of the sheet 2 after forming are maintained equidistant by the insertion of welded bars so as to maintain the shape of the structure during cooling. This cooling is slow cooling in the open air to minimize the residual stresses by slow relaxation. In one variant, the cooling may be performed in a chamber suitable for slow cooling.

After this cooling—and referring again to FIG. 1, machining (step 300) is performed on the structure curved in the forming step, in particular 3D milling with a 5-axis tool. Such a step makes it possible to form in the 70-mm thickness E of the sheet 2 reinforcing ribs 35, as illustrated in FIG. 3, in particular ribs forming a network of triangles of “isogrid” type. To perform quality 3D machining cycles, programming on a 5-axis milling machine 10 control panel is advantageously carried out.

One of the advantages of the invention is that it can render redundant the steps of straightening and/or pressing for the folding of sheets, or of rolling for bending. The sheet obtained according to the process of the invention is in principle directly to size after the hot forming, and the envisaged machining should not modify this state. The machining 300 may optionally be followed, if necessary, by a finishing step 400 (cf. FIG. 1) by rectification so as to obtain particular surfacing characteristics of the target structure. A step of straightening and of shaping by press, by shot-blasting or by any other shaping means may be envisaged in exceptional cases.

The structure in FIG. 4 a, a landing gear case 30 made in accordance with the invention, has “isogrid” ribs 35 a on the side walls 36 of the case and ribs 35 b on the roof 38 of the case, parallel to the plane of symmetry Ps of the case. Other structuring ribs 35 c form arches parallel to the entry 31 and exit 33 arches of the case, perpendicular to the plane of symmetry Ps.

Other examples of reinforced, curved monobloc structures are illustrated, respectively, by FIGS. 4 b to 4 d:

-   an airtight underside 40 (aircraft front end) with biaxial twofold     curvature along two perpendicular planes P1 and P2, having parallel     ribs 41 and 42 along these two perpendicular planes; -   a cockpit 50 having ribs 51 of uniaxial curvature essentially     parallel to the plane of symmetry Ps of the cockpit 50; -   a doorframe 60 with twofold curvature having ribs 61 and 62     respectively parallel to one and the other of these curvatures, and     also diagonal ribs 63.

Alternatively, in another embodiment, the machining step 300 of FIG. 1—consisting in machining the rolled primary form from the first step 100—precedes the forming step 200.

The conditions for performing the machining and the forming are adapted to the order in which these operations follow each other, in particular the cooling conditions after forming and the forming tools when the sheet is already machined.

The invention is not limited to the production examples described and represented. It is possible, for example, to perform any type of forming and of machining suited to the structure to be made, as a function of the larger or smaller size, rib network density or radius of curvature criteria to be achieved. Furthermore, the invention is not limited to the aeronautical field, and may be applied to any field, for example to the field of marine or terrestrial transport or to the construction field. 

1-13. (canceled)
 14. Process for making a reinforced metallic structure with uniaxial (Ps) or biaxial (P1, P2) curvature in monobloc form, characterized in that it consists in forming as heavy-gauge or plate-gauge sheet an aluminium alloy material containing scandium, and then in combining steps of forming (200) and of machining (300) of the said sheet, the forming step (200) giving the curvature(s) and the machining step (300) producing a network of reinforcing ribs (35, 41, 42, 51, 61, 62).
 15. Production process according to claim 14, in which the machining step (300) is performed after the forming step (200).
 16. Production process according to claim 14, in which the machining step (300) precedes the forming step (200).
 17. Production process according to claim 14, in which the machining step (300) is 3D and 5-axis machining.
 18. Production process according to claim 14, in which the forming (200) is hot forming.
 19. Production process according to claim 18, in which the hot-forming step is shaping with a punching (4) and stamping (6) tool heated to the same temperature as the sheet.
 20. Production process according to claim 19, in which the tool is hollow and/or made of a ceramic material.
 21. Production process according to claim 18, which the thermal contact between the die (6) and the sheet to be formed (2) is minimized by dimpling (6 c) at the edge (6 c) of the die (6).
 22. Production process according to claim 18, in which the hot forming (200) is performed by pressing the punching tools onto the die at a temperature of between 325 and 350° C.
 23. Production process according to claim 18, in which the hot-forming step (200) is completed by retraction of the stamping tools (4, 6) followed by slow cooling of the form obtained.
 24. Production process according to claim 14, in which the aluminium alloy is an Al—Mg—Sc alloy with proportions of magnesium and of scandium, respectively, less than 8% and 0.5%, preferably, respectively, between 2% and 7% and between 0.01% and 0.45%, and even more preferentially within the ranges 3-6% and 0.015-0.4%.
 25. Production process according to claim 14, which the process is finalized by a finishing step (400).
 26. Reinforced curved metallic structure of an aircraft subassembly, characterized in that it is made by performing the process according to claim
 14. 